This invention relates to information recording media in the form of light-readable information-bearing discs, and more particularly to such media which are made up of a front-side disc and a back-side disc secured to one another back to back to form a two-sided stack.
Light-readable information recording media in the form of information-bearing discs are well known as shown, for example, by Kramer U.S. Pat. No. 5,068,846. Commercially available compact discs ("CDS") and compact disc read-only memories ("CD-ROMs") are examples of recording media of this general type.
More recently, digital versatile discs ("DVDs") have emerged wherein light-readable information technology is employed to record movies and other similar real-time audio/visual programming (hereinafter "program information") on discs. To increase program information storage capacity, information-bearing discs are formed from two discs (similar in appearance to conventional audio CDS) bonded back to back (see, for example, Marquardt, U.S. Pat. No. 5,729,533, entitled "Two-sided, Light-Readable Information Recording Disc Stacks And Methods Of Making Same," the entire contents of which are hereby incorporated by reference).
Relatively long audio/visual programs will not fit on one disc of conventional audio CD size. As such, the "back-side" (i.e., the second disc bonded to the back of the "front-side" or first disc) of a information-bearing disc may often contain "spillover" audio/visual program information (i.e., information which would not fit on the front-side of the information-bearing disc).
During the formation of a one-program-information-side information-bearing disc (for example, a information-bearing disc wherein only the front-side disc in the two disc stack contains program information), a substantially transparent plastic disc (i.e., transparent enough to allow a light beam to remain detectable following round trip travel through the plastic disc) with a substantially planar first surface (i.e., planar enough to allow a light source to differentiate topographical features disposed on a surface of the plastic disc as the disc rotates) is provided. Opposite the first surface is a second surface having program information recorded on it by means of local changes in the distance of the second surface from the first surface. A light-reflecting coating (e.g., aluminum) is then deposited (e.g., sputtered) over the second surface, the light-reflecting coating conforming to the local changes of the second surface so that light is reflected back through the disc in accordance with the program information recorded by means of the local changes.
Once the front-side program-information-bearing disc has been fabricated, one-program-information-side information-bearing disc formation is completed by bonding a back-side non-program-information-bearing disc (e.g., a disc without program information such as a blank disc or a disc with artwork, labels, or other ornamentation) to the front-side disc. This type of non-program-information-bearing disc does not normally contain a light-reflecting coating so that bonding may be performed using a light-curable bonding material which is cured by transmitting light through the back-side plastic disc. Light can not be transmitted through the front-side disc because it contains a light-reflecting coating.
When a two-program-information-sides information-bearing disc (for example, an information-bearing disc wherein both discs contain program information) is formed, a light-reflecting coating must be deposited on the back-side disc so that program information recorded thereon may be read out. A light-reflecting coating may also be required if "pit art" (see previously incorporated Marquardt U.S. Pat. No. 5,729,533, entitled "Two-sided, Light-Readable Information Recording Disc Stacks And Methods Of Making Same"), "enhanced pit art" (see, for example, Parette et al. U.S. patent application Ser. No. 08/714,302, entitled "Methods for Providing Artwork on Plastic Information Discs," filed Sep. 18, 1996, which is hereby incorporated by reference herein), or similar visible image information is contained on the back-side disc. (As used herein, the term "information," unmodified, refers to both program information and visual image information such as pit art, enhanced pit art, holographic artwork, and other labels/ornamentation.)
Regardless of whether the information-bearing disc has information on one or both sides, it is important to the light-readability (i.e., playability) of the information-bearing disc that the front-side disc and back-side disc bond together to create a suitably flat information-bearing disc. Typically, when both front-side and back-side discs are formed by the same injection molding machine, both discs fit together to create a suitably flat information-bearing disc. However, due to slight variations between molding machines, front-side and back-side discs formed on different molding machines may not bond together to create a suitably flat information-bearing disc, causing rippling and buckling (this warping is also referred to as the "potato chip" effect) which reduces the light-readability of the information-bearing disc.
A need therefore exists for a method of fabricating information-bearing discs such that the effects of variations in molding machines are minimized. Such a method will allow bonding together of front-side and back-side discs to create a substantially flat information-bearing disc with improved light-readability.
In view of the foregoing, it is an object of this invention to provide substantially flat information-bearing discs and methods of fabrication.